Planck spots hot gas bridging galaxy cluster pair

ESA’s Planck space telescope has made the first conclusive detection
of a bridge of hot gas connecting a pair of galaxy clusters across 10
million light-years of intergalactic space. Planck’s main mission is a
survey of the remnant light from the Big Bang and the Max Planck
Institute for Astrophysics (MPA) in Garching has developed important
software components for the mission and takes part in the analysis and
scientific interpretation of the data.

Fig. 1:
Planck has discovered a bridge of hot gas that connects the galaxy
clusters Abell 399 (lower centre) and Abell 401 (top left). The galaxy
pair is located about a billion light-years from Earth, and the gas
bridge extends approximately 10 million light-years between them. The
image shows the two galaxy clusters as seen at optical wavelengths with
ground-based telescopes and through the Sunyaev-Zel'dovich effect (in
orange) with the Planck satellite.
Credits: Sunyaev–Zel’dovich effect: ESA Planck Collaboration; optical image: STScI Digitized Sky Survey

Movie:
This
movie
shows the three-dimensional structure of the cosmic web, the
wispy network of both dark and baryonic matter that is believed to
pervade the Universe. The movie was extracted from a numerical
simulation of the formation and evolution of cosmic structure. Galaxy
clusters are visible where the concentration of galaxies (shown in
white and purple) is higher. Clusters form in the densest knots of the
cosmic web, where filaments intersect. The density of gas in the
filaments that link the cluster is represented with different colours,
with dark brown indicating less dense regions and brighter colours
(from orange to yellow and green) indicating increasingly denser
regions. The colour scale also shows the gas temperature, ranging from
less than a million K in the darker regions up to several tens of
million K in the green regions.
Credit: Movie courtesy of Klaus Dolag, Universitäts-Sternwarte München, Ludwig-Maximilians-Universität München, Germany

Planck’s primary task is to capture the most ancient light of the
cosmos, the Cosmic Microwave Background, or CMB. As this faint light
traverses the Universe, it encounters different types of structure
including galaxies and galaxy clusters – assemblies of hundreds to
thousands of galaxies bound together by gravity. If the CMB light
interacts with the hot gas permeating these huge cosmic structures,
its energy distribution is modified in a characteristic way, a
phenomenon known as the Sunyaev–Zel’dovich (SZ) effect. This effect is
named after the two scientists who predicted it in 1969: Rashid
Sunyaev, presently director at the Max Planck Institute for
Astrophysics, and Yakov Zel'dovich.

The SZ effect has already been used by Planck to detect
galaxy clusters themselves,
but it also provides a way to detect faint filaments of gas that might
connect one cluster to another. Since the early universe, filaments
of gaseous matter pervade the cosmos in a giant web, with clusters
eventually forming in the densest nodes. Much of this tenuous,
filamentary gas remains undetected, but astronomers expect that it
could most likely be found between interacting galaxy clusters, where
the filaments are compressed and heated up, making them easier to
spot.

"Our numerical simulations of dark and baryonic matter agree pretty
well with the statistical distribution of galaxies and galaxy clusters
in the cosmic web," says Klaus Dolag, the scientists at the University
Observatory Munich and the Max Planck Institute for Astrophysics, who
provided the cosmological simulations for comparison to the
observational data. "The challenge now is to try and also match the
properties of the less dense filaments between the clusters, which are
much harder to observe."

Planck’s discovery of a bridge of hot gas connecting the clusters
Abell 399 and Abell 401, each containing hundreds of galaxies,
represents one such opportunity. The presence of hot gas between the
billion-light-year-distant clusters was first hinted at in X-ray data
from ESA’s XMM-Newton, and the new Planck data confirm the
observation.

“By combining Planck data with archival X-ray observations from the
German satellite ROSAT, we estimated the temperature of gas in the
bridge as about 80 million degrees Celsius,” notes Torsten Enßlin, a
Planck Collaboration scientist from the Max Planck Institute for
Astrophysics. Early analysis suggests the gas could be mixture of the
elusive filaments of the cosmic web mixed with gas originating from
the clusters. “It is still debated whether the gas derives from the
intra-cluster medium or from the gas that was previously part of the
two clusters; numerical simulations suggest that it could well be a
mixture of both,” explains Enßlin. “Further analysis of the complete
Planck data might help to clarify this issue by revealing additional
examples,” he adds.

The new finding highlights the ability of Planck to probe galaxy
clusters to their outskirts and beyond, examining their connection
with the gas that permeates the entire Universe and from which all
groups of galaxies formed.